1. Field of the Invention
The present invention relates to an inverter controlling dimming of a lamp employed in a liquid crystal display (LCD) backlight, and more particularly, to an LCD backlight inverter ultimately prolonging a life of a lamp by smoothly increasing or decreasing a current provided to the lamp in the LCD backlight when the lamp is controlled by burst dimming by using pulse width modulation (PWM).
2. Description of the Related Art
In general, an LCD is not capable of producing own light and thus employs a backlight for irradiating light from the back side to the front side thereof. To date, a cold cathode fluorescent lamp (CCFL) is mostly employed in a backlight for a large-sized LCD, together with an inverter for driving the lamp. In particular, since it is important to maintain a regulated level of brightness of the lamp employed in the backlight of the LCD, the inverter employs a constant current circuit to feed back the current of the lamp in order to maintain a regulated level of the current of the lamp. In addition, a dimming control circuit whose LCD brightness is capable of being externally adjusted according to the changes in the surrounding environment is applied. That is, an LCD backlight inverter requires a circuit for lighting the lamp at stable a brightness level set by a user.
FIG. 1 is a diagram illustrating a circuit regulating a constant current and controlling dimming in a conventional LCD backlight inverter.
As shown in FIG. 1, the conventional backlight inverter includes an error amplification unit 11 receiving a voltage corresponding to a current flowing through a lamp (the lamp current) from the lamp (not shown) as a feedback, comparing the feedback voltage with a predetermined reference voltage Vref1, and outputting a voltage corresponding to a difference; and a dimming control pulse generating unit 12 comparing an externally inputted dimming control signal with a predetermined period of triangle wave and generating a dimming control pulse with a duty controlled according to a comparison result.
The error amplification unit 11 includes an error amplifier 111 having an inversion input end to which the feedback voltage corresponding to the lamp current is inputted and a noninversion input end to which the reference voltage Vref is inputted; and a capacitor 112 connected between the inversion input end and an output end of the error amplifier 111. In addition, the dimming control pulse generating unit 12 includes a pulse width modulation (PWM) oscillator 121 generating the predetermined period of triangle wave; a first comparator 122 comparing the externally inputted dimming control signal S1 with the triangle wave S2 of the PWM oscillator 121 and generating the dimming control pulse S3 with a duty controlled according to a comparison result; and a switch 123 grounding an output end N1 of the error amplification unit 11 according to the dimming control pulse S3. In addition, the lamp control pulse generating unit 13 includes a main oscillator 131 generating a triangle wave S5; and a second comparator 132 comparing the triangle wave generated by the main oscillator 131 with an error signal S4 of the error amplification unit 11 controlled by the dimming control pulse and generating a lamp control pulse with a duty determined according to a comparison result.
FIG. 2 illustrates waveforms at a plurality of nodes in the conventional LCD backlight inverter having the above-described circuit structure. As shown in FIG. 2, the first comparator 122 of the dimming control pulse generating unit 12 compares the externally inputted dimming control signal S1 with the triangle wave S2 generated from the PWM oscillator 121 and outputs the dimming control pulse having a high state when a level of dimming control signal S1 is higher than a level of the triangle wave S2. Depending on a state of the switch 123 controlled by the dimming control pulse S3, the output part of the first error amplifier 11 is grounded. That is, the switch is turned on in a high state of the dimming control pulse S3 so that the output end N1 of the first error amplifier 11 is grounded, and the switch 123 is turned off in a low state of the dimming control pulse S3 so that the output signal of the first error amplification unit 11 is transmitted to the input end of the lamp control pulse generating unit 13. The signal transmitted to the input end of the lamp control pulse generating unit 13 is denoted by S4.
The input signal of the lamp control pulse generating unit 13 and the triangle wave S5 generated from the main oscillator 131 are compared by the second comparator 132, and the lamp control pulse S6 is outputted with a duty controlled according to a comparison result.
In such a conventional LCD backlight inverter, since dimming is controlled as the switch 123 is instantaneously turned on and off, the current transmitted to the lamp is a current with a rectangular envelope curve, having a large value and instantaneously transmitted or interrupted as shown in FIG. 2. That is, a magnitude of the lamp current is determined by duty of the lamp control pulse outputted from the second comparator 132. Since the duty of the lamp control pulse is determined according to the input signal of the second comparator 132 having high and low states by instantaneously turning on and off the switch 123, the lamp control pulse has a regulated level of duty. Therefore, the lamp current suddenly changes to a value corresponding to the duty of the lamp control pulse.
This sudden change of the lamp current, however, adversely affects the lamp, such as shortening the life of the lamp and so on. Further, the sudden change of the lamp current may cause noises in the transformer included in the LCD inverter circuit.